The present invention relates generally to the large scale production of anhydrous nitric acid and nitric acid solutions of dinitrogen pentoxide and more particularly to an electrolytic method and apparatus for simultaneously synthesizing water-free nitric acid and solutions of dinitrogen pentoxide in anhydrous nitric acid.
Nitric acid has become a major industrial chemical, with diverse applications and large scale industrial use in the manufacture of fertilizers, organic chemicals, explosives and the like. Generally, for most industrial and other applications, aqueous nitric acid is produced at a concentration of 50-70 wt. % HNO.sub.3 by a standard ammonia oxidation process. In this process, ammonia is oxidized with excess oxygen over a catalyst to form nitric oxide and water. The nitric oxide is then oxidized to nitrogen dioxide, which is absorbed in water to form nitric acid and additional nitric oxide. The nitric acid is then concentrated, but since HNO.sub.3 forms an azeotrope with water at 68.8 wt. %, it cannot be separated from the water or concentrated beyond approximately 70 wt. % by simple distillation.
While the commonly available 70 wt. % HNO.sub.3 is suitable for the production of ammonium nitrate fertilizer and many other inorganic chemicals, more highly concentrated or completely anhydrous (water-free) nitric acid is required for use in many organic nitrations. Mixtures of nitric and sulfuric acids are also commonly used for organic nitrations, to insure a low water concentration which is favorable for these reactions. The rocket-fuel and semiconductor industries employ red fuming nitric acid, which typically consists of 15 wt. % dinitrogen tetroxide (N.sub.2 O.sub.4), 2 wt. % H.sub.2 O, and 83 wt. % HNO.sub.3.
Highly concentrated nitric acids are widely employed in the explosives industry. The prior known Bachman process, used commercially in the U.S. for the production of cyclonite (1-3-5-trinitro-1,3,5,-triazine or commonly known as RDX) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), involves a continuous nitration of hexamethylenetetramine by reaction with strong nitric acid, ammonium nitrate, and acetic anhydride. In England, RDX is manufactured by the Woolwich process in which hexamethylenetetramine is reacted with anhydrous nitric acid.
Anhydrous nitric acid, e.g., 98 to 100 wt. % HNO.sub.3, has been synthesized by distillation of a weaker aqueous solution of nitric acid with sulfuric acid, the latter serving as a dehydrating agent. Typically, 60 wt. % HNO.sub.3 is mixed with 93 wt. % H.sub.2 SO.sub.4 in a packed tower which is provided with a steam heated reboiler. The nitric acid vapor is distilled and condensed, and the sulfuric acid and water leave the bottom as approximately 70 wt. % H.sub.2 SO.sub.4. Water is then removed from the sulfuric acid in a sulfuric acid concentrator, and the 93 wt. % H.sub.2 SO.sub.4 is recycled in the process. An alternative extraction medium is a 72 wt. % solution of magnesium nitrate in water. In this process, which is used in conjunction with the U.S. manufacture of RDX and HMX, the nitrate solution typically leaves the distillation column at approximately 68 wt. %, and is reconcentrated by flashing to a steam heated vacuum drum.
These methods for producing anhydrous HNO.sub.3 require the recycling of large quantities of sulfuric acid or magnesium nitrate. This inherently presents the potential of major catastrophic accidents, as well as the production of large quantities of waste heat and energy. Thus from a cost standpoint, these processes are inherently deficient.
When water is further removed from anhydrous nitric acid, the anhydride of nitric acid, dinitrogen pentoxide (N.sub.2 O.sub.5), is formed as represented by the equation: EQU 2HNO.sub.3.fwdarw.N.sub.2 O.sub.5 +H.sub.2 O (1)
Thus solutions of N.sub.2 O.sub.5 in HNO.sub.3 can be prepared which can be thought of as greater than 100% HNO.sub.3, and which have unique properties for some chemical syntheses.
A number of organic nitration and nitrolysis reactions have been found to proceed faster, more efficiently, and in highest yield by the use of solutions of N.sub.2 O.sub.5 in HNO.sub.3. The high explosive HMX can be prepared by the reaction of a series of 1,3,5,7-tetra-azacyclooctanes with N.sub.2 O.sub.5, formed in the reaction mixture in-situ by dehydration of the nitric acid. The dehydration is accomplished by reagents such as phosphorus pentoxide (P.sub.2 O.sub.5), polyphosphoric acid, trifluoroacetic acid anhydride, or sulfur trioxide (SO.sub.3). Pure N.sub.2 O.sub.5 can also be synthesized by oxidation of N.sub.2 O.sub.4 with ozone. In a carbon tetrachloride medium, N.sub.2 O.sub.5 converts aliphatic secondary amines into nitramines in excellent yield. These reactions have never achieved large-scale use because of the high cost of producing N.sub.2 O.sub.5, either by chemical dehydration or by ozonolysis. Chemical dehydration requires expensive recycling processes, and ozonolysis is electrically inefficient.
A third general approach to the synthesis of nitric-acid solutions of N.sub.2 O.sub.5 is direct electrochemical oxidation of a suitable precursor compound.
The basic reaction, the oxidation of N.sub.2 O.sub.4 in HNO.sub.3 at a platinum anode in an electrolysis cell divided by a diaphragm, was first described in German Patent No. 231,546,
J. Zawadski and Z. Bankowski, Roznicki Chemii. 22 (1948), 233, extended this work, employing the same reaction and essentially the same type of apparatus, a laboratory size, stirred electrolysis cell. The anode comprised a platinum sheet, the cathode was made of sheet lead, and the diaphragm employed was a porous ceramic. In this method also, the cell voltage was controlled, and N.sub.2 O.sub.5 was produced with a current efficiency of 35% and a specific energy of 5 kWH/kg.
The electrolysis reaction that produces N.sub.2 O.sub.5 can be written as follows: EQU N.sub.2 O.sub.4 +2HNO.sub.3.fwdarw.2N.sub.2 O.sub.5 +2H++2e.sup.- (2)
If there is water in the nitric acid at the beginning of the electrolysis, it is be consumed by the reaction: EQU H.sub.2 O+N.sub.2 O.sub.5.fwdarw.2HNO.sub.3 (3)
At a certain point in time during the electrolysis, when all of the water has been consumed, the anolyte consists solely of HNO.sub.3 (anhydrous) and unreacted N.sub.2 O.sub.4. From that point on, excess N.sub.2 O.sub.5 is generated. Eventually all of the N.sub.2 O.sub.4 is consumed by electrolysis, and the anolyte will then consist solely of HNO.sub.3 and N.sub.2 O.sub.5. If desired, the N.sub.2 O.sub.5 /HNO.sub.3 solution can be reacted with an aqueous nitric acid solution in the correct stoichiometric amount to yield pure, anhydrous HNO.sub.3.
In addition to Reaction 2, N.sub.2 O.sub.5 can also be formed by the electrolytic oxidation of HNO.sub.3 according to the reaction: EQU 2HNO.sub.3.fwdarw.N.sub.2 O.sub.5 +2H++(1/2)O.sub.2 +2e.sup.- (4)
The oxidation of HNO.sub.3 proceeds at a higher anode potential than Reaction 1 and may proceed concurrently with Reaction 1, if the anode potential is in a region where both can occur. Although Reaction 4 produces N.sub.2 O.sub.5, it also produces oxygen as a byproduct and the current efficiency for the production of N.sub.2 O.sub.5 is lower.
The current efficiency for N.sub.2 O.sub.5 production and yield based on the use of N.sub.2 O.sub.4 could be substantially improved compared to that obtained by Zawadski and Bankowski by performing the electrolysis in a controlled-potential electrolysis cell, and controlling the anode potential to minimize the extent of the oxidation of HNO.sub.3 (Reaction 4). See U.S. Pat. Nos. 4,432,902, 4,443,308 and 4,525,252. With the apparatus and methodology described in the aforementioned U.S. Patents, using a laboratory-size divided cell having a porous-glass membrane and a platinum-wire anode and cathode, a current efficiency of approximately 65% and a chemical yield of about 50% were achieved.
That anhydrous HNO.sub.3 can be produced by the electrolytic reactions described above, is also disclosed by USSR Patents Nos. 1,059,023A and 1,089,172A, but there is no discussion of the preparation of N.sub.2 O.sub.5 /HNO.sub.3 solutions per se in these patents. In the work described in the '023A patent, the oxidation of aqueous HNO.sub.3 was carried out according to Reaction 4 until anhydrous HNO.sub.3 is produced. In the work described in the '172A patent, N.sub.2 O.sub.4 was included in the aqueous nitric acid solution, and it was shown, as discussed earlier, that its oxidation increased the current efficiency of the process, because the contribution of Reaction 4 to the consumption of the current is reduced.
In carrying out the aforementioned reactions, a laboratory-size divided cell, with a ceramic diaphragm and a stainless steel cathode were used, but the anode potential was not controlled. Anode materials tested were platinum, glassy carbon, and metal-oxides on a titanium substrate. These oxides were RuO.sub.2, PbO.sub.2, MnO.sub.2, and Co.sub.2 O.sub.3.
The prior art cited above, relating to the production of N.sub.2 O.sub.5 /HNO.sub.3, has demonstrated the basic principles of the reactions and their feasibility on a laboratory scale, but none of the teachings has demonstrated how the electrochemical synthesis can be carried out on a large-scale, using the type of technology which would be suitable on an industrial, production scale.
Some of the problems encountered in translating the findings of laboratory experiments to large industrial scale production are summarized hereinbelow.
The electrolysis cells described in the prior art are stirred-reactors of less than 500-mL solution capacity. The removal of ohmic heat from the electrolyte is required during the electrolysis, and this cannot be done simply or efficiently with large stirred reactors without considerable design alterations.
The electrolysis cell container and diaphragm materials used in the laboratory are generally made of glass or ceramic. The fragility of these materials precludes their use as construction and diaphragm materials on a large scale basis. The extremely corrosive nature of the solutions used in this large scale process also severely limits the choices of compatible materials.
Noble-metal or noble-metal-oxide electrodes are best suited for use as the cell anode in these laboratory size experiments. However, because of the capital costs involved in their use, these materials can be used on a large scale only in the form of coatings, which, for the large scale production process remain largely untested.
Finally, electrolysis by the technique of controlling a 3-electrode controlled-potential is not practiced on a large scale because of the difficulty in the maintenance of the reference electrode and the lack of high-power automatic potentiostats. A different approach to supplying power to the electrolysis and potential control is required.
There is also no teaching in the prior art of suitable methods for on-line, real time, chemical analysis of the solutions during the electrolysis, especially methods for the measurement of the concentrations of N.sub.2 O.sub.4 and N.sub.2 O.sub.5, which are required for large-scale implementation of the basic process. In particular, the concentrations of both N.sub.2 O.sub.4 and N.sub.2 O.sub.5 must be known in order to control the proportions of the product solution for subsequent organic syntheses or for the preparation of anhydrous HNO.sub.3.
There is also no teaching in the prior art of any large-scale, electrolytic preparation of anhydrous HNO.sub.3 and N.sub.2 O.sub.5 /HNO.sub.3, nor on the concurrent production of N.sub.2 O.sub.4 by the cathode in the same electrolyzer or electrolysis cell. In an electrolysis cell for the preparation of N.sub.2 O.sub.5, the initial catholyte solution is generally either nearly-anhydrous nitric acid or aqueous nitric acid. The cathode reaction can be written as:
2HNO.sub.3 +2H.sup.+ +2e.sup.-.fwdarw.N.sub.2 O.sub.4 +2H.sub.2 O (5)
This reaction, in combination with Reaction (2) yields the net electrolysis-cell reaction, which is the same as Reaction (1), the dehydration of nitric acid. Thus in a perfectly operating electrolysis cell, the quantity of N.sub.2 O.sub.4 generated at the cathode would be exactly balanced by the quantity consumed at the anode. The N.sub.2 O.sub.4 generated in the catholyte could be recycled as a feedstock or as a makeup for the N.sub.2 O.sub.4 consumed in the anolyte, if a suitable method could be developed therefor. Recovery of the catholyte N.sub.2 O.sub.4 is essential for the economic operation of the process on a large scale. Flow electrolyzers with electrolyte recirculation through heat exchangers would be the design of choice for large-scale electrosynthesis.
It would be desirable, therefore, to have available, an economical method for the production of N.sub.2 O.sub.5 on a large scale, particularly in view of the fact that the selection of materials for the electrode coatings, the cell separator, and the cell frames, the methods of solution handling, and the techniques of chemical monitoring are critical to a realization of the scale-up for N.sub.2 O.sub.5 and HNO.sub.3 synthesis.
Accordingly, an object of the present invention is the large-scale, electrolytic production of anhydrous nitric acid.
Another object is the large-scale production of solutions of N.sub.2 O.sub.5 in HNO.sub.3.
Yet another object of the invention is an apparatus and methodology for the large scale production of anhydrous nitric acid and solutions of N.sub.2 O.sub.5 in HNO.sub.3.
Still another object is the recovery of the N.sub.2 O.sub.4 produced in the catholyte.
A further object of the invention is the recycling of the N.sub.2 O.sub.4 generated in the catholyte as a feedstock for the anolyte.
Still another object is to provide a method for monitoring the composition of the solutions during production.
Another object is the use of a plate-and-frame, flow through, divided-cell type electrolyzer With electrodes of aluminum coated with IrO.sub.2 and niobium coated with Pt or Pt-Ir, and ion-exchange or porous separators, for the production of N.sub.2 O.sub.5 /HNO.sub.3.
Another object is a method for monitoring the composition of the solutions during production.
Additional objects, advantages and novel features of the invention, together with additional features contributing thereto and advantages accruing therefrom will be apparent to those skilled in the art, from the following description of the invention which is shown in the accompanying drawings which are incorporated herein by reference thereto and form an integral part hereof. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.